The present invention generally relates to packaging apparatus and methods and, more particularly, to shrink wrapping apparatus and methods in which a plastic film shrinks tightly around one or more products.
Various methods and apparatus have been employed to shrink a plastic film about one or more products during an assembly line packaging operation. The products may be in many different forms and package configurations. Most notably, these apparatus typically comprise forced air ovens or other tunnel structures through which a conveyor passes. The products, which are encased in a loose plastic shrink film and then placed on the conveyor, pass through the oven or tunnel structure. As they are heated, for example, by a forced air system or other types of heaters, the film shrinks tightly about the product or products. The timing is such that the shrink process occurs as the product or products are traveling from the entrance to the exit of the oven or tunnel.
Ideally, the shrink wrap will uniformly shrink about the outer surface of the product or products with minimal distortion of the film. However, this is especially difficult with products that have irregular shapes. With such products, the shrink film may contact certain areas of the outer surface of the product and may not contact other areas of the outer surface. The problem is further complicated by the fact that many ovens and tunnels have xe2x80x9chot spotsxe2x80x9d and xe2x80x9ccold spotsxe2x80x9d due to uneven heat distribution within the oven or tunnel interior. For example, ovens that use forced air typically introduce the air into the oven interior through a duct and use various baffles which may be adjustable in an attempt to uniformly distribute the heated air throughout the interior of the oven. This adjustment procedure is often more of an art than a science and, especially when faced with small production runs, the adjustment process becomes even more difficult, inefficient, time consuming and therefore costly. Adjustments must properly balance the temperature, conveyor speed, and air flow through the various ducts and baffle structure. Often, this adjustment process can take up to several hours before achieving consistent shrink wrapped products of a quality acceptable to the customer. As mentioned above, this time consumption is especially impractical and costly for small production runs.
One specific aspect of prior ovens or tunnels which presents certain problems is the riser bar used to raise a short section of the conveyor within the oven or tunnel. These riser bars are used to briefly lift each product during its travel through the oven or tunnel as an aid to more uniformly distribute and control the bubble or balloon-like effect that briefly occurs in the shrink film during the shrink wrap process. As the product is briefly lifted off the conveyor at the riser bar location, the weight of the product is taken off the underlying shrink film. This reduces or eliminates grid marks in the film otherwise caused by the conveyor. Prior riser bars suffer from two general drawbacks. First, the conveyor rubs over the riser bar and, eventually, the friction wears down the riser bar making it less useful and in need of replacement. Second, the riser bar is of fixed diameter and, therefore, may work effectively only across a limited variety of product sizes and configurations run through the tunnel.
To address these concerns as well as other concerns in the shrink wrap field, it would be desirable to provide a shrink tunnel requiring little or no adjustments to be made to achieve high quality shrink wrapping of a wide range of product configurations and sizes. It would also be desirable to provide a shrink tunnel having an adjustable riser member allowing the elevation of a section of the conveyor to be changed in accordance with different product configurations, sizes and weights.
Generally, the invention relates to an apparatus for shrink wrapping at least one product and including a housing having an interior space with a top, a bottom, an entrance and an exit. The product may or may not be included in another container or package during the shrink wrap process. A heater is thermally coupled to the interior space and a first air mover can be operatively coupled with the interior space and configured to move air generally around the interior space and through the open area of the conveyor belt. A conveyor belt passes through the interior space from the entrance to the exit and preferably comprises an open configuration, such as a mesh configuration.
In one main aspect of the invention, an adjustable riser member is coupled to the conveyor belt and is adjustable in height to raise and lower a transverse section of the conveyor belt. For example, the transverse section of the conveyor belt may be raised higher for larger and/or heavier products and may be raised to a lesser extent for smaller and/or lighter products. For very lightweight products or products which are more prone to tip over, the riser member may be adjusted so that the conveyor remains flat. In the preferred embodiment, the riser member is a bar or shaft extending transverse to the conveyor belt, and optionally including at least one roller engaging the conveyor belt. As another desirable feature, the riser member is adjustable from outside the housing for ease of use by the operator. The conveyor belt is preferably a free tension belt, i.e., one that is mounted and moves under little or no tension. This ensures that the belt height may be easily adjusted without placing the conveyor belt or the riser member under undesirable stress. It will be appreciated that this feature may be incorporated into any type of shrink wrap apparatus.
The openly configured conveyor belt is substantially unobstructed on upper and lower sides thereof as it travels within the housing. For example, the conveyor belt may be supported by the riser member or bar extending across one generally central section thereof transverse to the conveyor path and may also be supported by thin underlying rods.
In accordance with the preferred embodiment, respective upper and lower heaters are located above and below the conveyor belt and are constructed generally as assemblies with the air movers which may comprise fan blades. In another aspect of the invention, the fan blades have at least substantially no pitch. For that reason, the fans do not forcefully direct air at the products moving along the conveyor belt but rather move the air generally uniformly and with relatively low velocity within the interior space of the tunnel. This helps to ensure that no xe2x80x9chot spotsxe2x80x9d or xe2x80x9ccold spotsxe2x80x9d are created within the interior space of the tunnel.
In accordance with another aspect of the invention, the conveyor belt is positioned closer to a mid-point between the upper heater and the lower heater than to the upper and lower heaters themselves. Due to this generally central positioning of the conveyor belt between the upper and lower heaters, the products moving along the conveyor belt are more uniformly heated on their upper and lower sides. The same spatial relationship also preferably exists between the top and bottom walls of the tunnel housing and the conveyor. This general aspect of the invention furthers the goal of shrinking the plastic film about the product or products without burning the film or producing unacceptable distortions of the film.
The present invention further contemplates various methods of shrink wrapping at least one product including the various operations of the tunnel described hereinabove either taken alone or in any of their various combinations in accordance with the needs of the user.
These and other features, objects and advantages will become more readily apparent to those of ordinary skill in the art upon review of the following detailed description taken in conjunction with the accompanying drawings.